1. Field of the Invention
The present invention relates to an apparatus for converting an arc furnace installation from three-phase operation to d.c. operation, wherein the arc furnaces includes a high-voltage transformer, a controllable three-phase furnace transformer, a furnace casing, and at least two electrodes.
2. Description of the Prior Art
The company brochure "Arc furnace installations Brown Boveri Arcmelt Type AM" of Messrs. Aktiengesellschaft Brown, Boveri & Cie., Baden/Switzerland, publication No. CH-IH 512434E (undated), in particular FIGS. 7 and 8 on pages 12 and 13, shows the conventional electrical supply system for a three-phase arc furnace.
Three-phase arc furnaces for the production of electric steel have been used for more than half a century. In this period, this type of furnace has been developed technically further and further and, in addition to the induction furnace, it is nowadays regarded as the most important electrical melting apparatus in the iron and steel industry.
Particularly in the melt-down phse, when the electrodes are in contact with feed material not yet molten, three-phase arc furnaces frequently cause short-circuits which are transmitted as a reaction to the supply network, in spite of very quickly responding electrode-adjustment systems. These reactions on the network--the so-called flickers--can lead to voltage dips in low-power supply networks, with detrimental consequences for the consumers connected to the same network.
Three-phase arc furnaces also tend to fluctuating arc lengths. This phenomenon, on the one hand, unfavourably influences the electrode consumption and puts a high thermal load on the refractory lining and, on the other hand, leads to a noise nuisance for the furnace operating crew.
The advances in the development of semi-conductor components in recent years have provided an incentive for conceiving d.c. arc furnaces for industrial use in the smelting of electric steel.
In introducing the d.c. arc furnace, as a melting apparatus replacing the three-phase arc furnace into the iron and steel industry, the main aim was to retain the proven properties of the arc furnace--economy, reliability and robustness--but to eliminate the abovementioned disadvantageous features of the three-phase arc furnace.
When any investment decisions on arc furnace installations have to be made, the iron and steel industry is confronted by two alternatives:
Either
New investment in a d.c. arc furnace installation or
Conversion of an already existing three-phase arc furnace installation to d.c. operation, while retaining usable, already existing parts of the installation and newly procuring only the constructional components required in addition.
Since nowadays three-phase arc furnaces are almost exclusively in operation, great importance must be attached to the second alternative, and this is certainly true from the point of view of economic considerations.
The initial situation, when converting an existing three-phase arc furnace installation to d.c. operation, is essentially as follows:
Retention of the controllable three-phase furnace transformer already existing,
Retention of the high-voltage installation associated with the furnace transformer,
New procurement of a rectifier arrangement,
New procurement of the high-current side of the arc furnace equipment, and
New procurement or modification of the arc furnace itself, which essentially consists of the furnace casing and furnace cover, electrode-control device, and the rest.
When converting an existing three-phase arc furnace installation to d.c. operation, the obvious course in itself would be to insert a rectifier arrangement, for example a diode bridge circuit, by means of which the three-phase operation could be converted into d.c. operation, between the three-phase furnace transformer and the arc furnace. However, this measure would not lead to an operable d.c. arc furnace, in particular for the following reasons:
The inductive reactance of the high-current lines of the arc furnace, which represents a significant proportion of the total inductive reactance in three-phase operation and makes a significant contribution to the limitation of the furnace short-circuit current, then disappears in d.c. operation, and the ratio of the short-circuit current to the operating current of the furnace is increased in an inadmissable manner.
On the high-current side, the windings of the three-phase furnace transformers are arranged as open data connections which are closed outside the transformer. Between the furnace electrode and the melt or the solid constituents of the feed material, this results, in the furnace casing, in a voltage which amounts to ##EQU1## times the secondary voltage of the furnace transformer.
The arc voltage in turn is ##EQU2## times the secondary voltage, at the maximum furnace power. In combination, this means that, in three-phase operation, the arc voltage at maximum furnace power is smaller than the secondary voltage of the furnace transformer by a fractor of ##EQU3##
It is now an extremely important condition that the arc voltage for a three-phase furnace converted to d.c. operation, under the assumption of the same furnace size and power as in the previous three-phase operation, should be approximately the same as in the previous there-phase operation.
The mere insertion of only a rectifier arrangement between the three-phase transformer and the arc furnace wouild then increase the arc voltage in an inadmissable manner. A long arc with high gradient intensity would form, and this, as already described above, would have damaging effects on the refractory lining in the furnace casing.